The instant invention pertains to hydrogen storage alloys as well as to electrochemical cells, batteries and fuel cells using these alloys. More particularly, the instant invention relates to hydrogen storage alloys having microstructures that are highly permeable and/or that include high concentrations of catalytically active metal or metal alloy particles. Most particularly, the instant invention relates to hydrogen storage alloys suitable for use as negative electrode materials in metal hydride batteries that exhibit high powers and high discharge rates at low operating temperatures.
Consumer and industrial applications continue to drive demand for new and efficient batteries for use as energy sources. Important goals include obtaining ever more power from increasingly smaller battery packages in an environmentally respectful fashion. Envisioned applications for batteries include everything from mobile electronics to electric vehicles. Portability, rechargeability over a large number of cycles, low cost, high power, lightweight and consistent performance over widely varying loads are among the key attributes required for batteries. The specific combination of battery performance requirements varies widely with the intended application and the battery components and materials are typically optimized accordingly. An important developing application area for rechargeable batteries is electric vehicles (EV) and hybrid electric vehicles (HEV). In these applications, the battery must have the ability to provide high currents in short time periods in order to achieve effective acceleration. High discharge rates are therefore necessary. High battery power over extended time periods are also needed so that vehicles of reasonable size and weight can be maintained in motion for reasonable time intervals without recharging. Rapid recharging over many cycles should also be possible using readily available electrical power sources. The preferred cycle life profile also requires a high number of charge/discharge cycles at a low, rather than high, depth of discharge. Progress has been made in the development of batteries for HEV applications and two HEV automobiles have recently been made available to the U.S. public. Nonetheless, the batteries used in these automobiles represent compromises and trade-offs in relevant performance parameters and new developments are needed to further the capabilities of HEV and EV products.
One aspect of rechargeable batteries for HEV, EV, 42 V SLI and other applications that has received relatively little attention is low temperature operation. For HEV and EV products it is desirable to have batteries that perform well in winter climates. Similarly, achievement of portable and stationary power sources based on rechargeable batteries that are capable of functioning outdoors in cold climates or in indoor cold environments is also desirable. A basic limitation of virtually every battery technology is a diminution of power and performance at low temperature. The deleterious effects of temperature are especially pronounced below freezing.
Nickel metal hydride batteries have emerged as the leading class of rechargeable batteries and are replacing earlier generation nickel-cadmium batteries in many applications. Current HEV and EV products, for example, utilize nickel metal hydride batteries and expanded performance of HEV and EV products in the future are expected to depend largely on the capabilities of nickel metal hydride batteries. Like other rechargeable batteries, nickel metal hydride batteries suffer significant degradation in power and performance upon a lowering of temperature.
Improvements in the low temperature performance require consideration of the underlying components and principles of operation of nickel metal hydride batteries.
Nickel metal hydride batteries typically include a nickel hydroxide positive electrode, a negative electrode that incorporates a metal containing hydrogen storage alloy, a separator and an aqueous alkaline electrolyte. The positive and negative electrodes are housed in adjoining battery compartments that are typically separated by a non-woven, felled, nylon or polypropylene separator. Several batteries may also be combined in series to form larger battery packs capable of providing higher powers, voltages or discharge rates.
The charging and discharging reactions of nickel metal hydride batteries have been discussed in the art and may be summarized as shown below:
Charging:
positive electrode: Ni(OH)2+OHxe2x86x92NiOOH+H2O+exe2x88x92
negative electrode: M+H2O+exe2x88x92xe2x86x92MH+OHxe2x88x92
Discharging
positive electrode: NiOOH+H2O+exe2x88x92xe2x86x92Ni(OH)2+OHxe2x88x92
negative electrode: MH+OH W M+H2O+exe2x88x92
Much work has been completed over the past decade to improve the performance of nickel metal hydride batteries. Optimization of the batteries ultimately depends on controlling the rate, extent and efficiency of the charging and discharging reactions. Factors relevant to battery performance include the physical state, chemical composition, catalytic activity and other properties of the positive and negative electrode materials, the composition and concentration of the electrolyte, the separator, the operating conditions, and external environmental factors. Various factors related to the performance of the positive nickel hydroxide electrode have been considered, for example, in U.S. Pat. Nos. 5,348,822; 5,637,423; 5,905,003; 5,948,564; and 6,228,535 by the instant assignee, the disclosures of which are hereby incorporated by reference. Work on suitable negative electrode materials has focused on intermetallic compounds as hydrogen storage alloys since the late 1950""s when it was determined that the compound TiNi reversibly absorbed and desorbed hydrogen. Subsequent work has shown that intermetallic compounds having the general formulas AB, AB2, A2B and AB5, where A is a hydride forming element and B is a weak or non-hydride forming element, are able to reversibly absorb and desorb hydrogen. Consequently, most of the effort in developing negative electrodes has focused on hydrogen storage alloys having the AB, AB2, AB5 or A2B formula types.
Desirable properties of hydrogen storage alloys include: good hydrogen storage capabilities to achieve a high energy density and high battery capacity; thermodynamic properties suitable for the reversible absorption and desorption of hydrogen; low hydrogen equilibrium pressure; high electrochemical activity; fast discharge kinetics for high rate performance; high oxidation resistance; weak tendency to self-discharge; and reproducible performance over many cycles. The chemical composition, physical state, electrode structure and battery configurations of hydrogen storage alloys as negative electrode materials in nickel metal hydride have been investigated and reported in the prior art. Some of this work is described in U.S. Pat. Nos. 4,716,088; 5,277,999; 5,536,591; 5,616,432; and 6,270,719 to the instant assignee, the disclosures of which are hereby incorporated by reference.
Efforts to date indicate that intermetallic compounds are capable of effectively functioning as negative electrode materials in rechargeable batteries, but that important properties are difficult to optimize simultaneously. Hydrogen storage alloys of the AB5 type, for example, generally have high initial activation, good charge stability and relatively long charge-discharge cycle life, but at the same time have low discharge capacity. Furthermore, attempts to increase the cycle life generally lead to reductions in the initial activation. Hydrogen storage alloys of the AB2 type, on the other hand, typically possess high discharge capacity, but low initial activation and relatively short cycle life. Efforts to improve upon the initial activation generally come at the expense of cycle life. Other important properties include discharge rate, discharge current, and constancy of output over time. It has proven difficult in most applications to simultaneously optimize all desired battery attributes and as a result, compromises are normally made in which some properties are sacrificed at the expense of others.
Efforts to universally improve as many of the desirable performance attributes of hydrogen storage alloys as possible require a molecular level consideration of the structural and interatomic interactions of the materials used as hydrogen storage alloys. One of the instant inventors, S. R. Ovshinsky, has formulated a novel and versatile strategy for designing materials having new and/or expanded functionality. A key concept advanced by Ovshinsky is the appreciation of the new and varied degrees of freedom afforded by the disordered and amorphous states of matter. Ovshinsky recognized that the ordered crystalline lattice imposed many constraints on the structure and properties of materials due to a rigid adherence of atoms to a prescribed structural lattice and instead embraced the disordered and amorphous states for the enormous flexibility in chemical bonding, intermolecular interactions and structural configurations that they provide. Ovshinsky viewed disordered and amorphous materials in terms of constituent local structures, each of which has unique properties according to the chemical elements and topology present, which collectively and synergistically interact to produce macroscopic materials having novel structures and properties. Heretofore unachievable macroscopic properties become possible through the judicious assembly of properly tailored constituent local structures.
Through his viewpoint, Ovshinsky discovered, elucidated and developed the principles of atomic engineering, chemical modification and total interactive environment that have revolutionized the ways in which people view and understand materials and their properties. According to these principles, the structure and properties of materials are strongly interrelated and new material properties necessarily flow from new structural degrees of freedom. Ovshinsky realized that crystalline solids, with their prescribed and rigid structures, were simply incompatible with the goal of designing new materials with new functionality. On the contrary, only the disordered and amorphous states permit the structural flexibility, through control of the local chemical compositions, topology and assemblage of constituent local structures, necessary to achieve a broad new concept of materials design.
Implementation of the Ovshinsky principles in the prior art has emphasized amorphous and disordered inorganic materials such as tetrahedral amorphous semiconductors (e.g. Si, Ge), trivalent, sheet like systems formed from elements of Group V of the periodic table (e.g. As), and divalent, chain and/or ring systems formed from elements of Group VI of the periodic table (e.g. Se, Te, S). Representative applications of the Ovshinsky principles to materials based on elements selected from Groups IV, V and VI are included in U.S. Pat Nos. 4,177,473 and 4,177,474 to Ovshinsky; the disclosures of which are hereby incorporated by reference. In these patents, Ovshinsky teaches the use of modifier materials to control the electrical activation energy and conductivity of inorganic materials by modifying defect electronic configurations through orbital interactions of one or more modifying elements with microvoids, dangling bonds, nearest neighbor interactions, or lone electron pairs. These orbital interactions provide effects such as charge compensation, polyvalency, lone pair compensation, three center bonding, and lone pair-lone pair influences that lead to the formation of new electronic states and/or deactivation of native electronic states that act to determine the Fermi level and electrical activation energy. Inclusion of one or more modifier elements constituted a means for perturbing local composition, topology and intermolecular interactions in such a way as to produce the structural deviations of the unmodified material necessary to achieve a preferential level of conductivity. Orbital interactions of the modifiers with the surrounding disordered or amorphous material permitted the establishment of local structures and bonding configurations that are not possible in the crystalline state. By controlling the amount and chemical identity of the modifying element, Ovshinsky demonstrated variability of electrical conductivity over a range spanning more than ten orders of magnitude.
In U.S. Pat. Nos. 4,520,039 and 4,664,960, the disclosures of which are hereby incorporated by reference, Ovshinsky further teaches the inhomogeneous arrangement of constituent local structures in disordered and amorphous materials. Inhomogeneous disordered and amorphous materials include local structures whose chemical compositions, topology and orbital interactions are non-uniform over macroscopic length scales throughout a material. Inhomogeneity provides further opportunities to increase the range of structures, and hence properties, available from a material because it provides a means for selectively controlling the placement of atoms and their nearest neighbor interactions to produce a tailored distribution of chemical and topological environments within a material. A disordered or amorphous material that is homogeneous, on the contrary, benefits from chemical and topological flexibility locally, but necessarily includes an implicit constraint in that homogeneity requires repetition of local chemical composition and topology within some length scale over macroscopic distances. Inhomogeneity further aids the designer of materials because individual chemical and topological environments may be mixed, matched and assembled at will to achieve macroscopic materials having a wider array of properties. Inhomogeneity, for example, permits formation of materials in which the electronic, magnetic, chemical and phonon properties of a material may be selectively coupled or decoupled to one another. Graded materials are one example of inhomogeneous materials.
A need exists for improved rechargeable batteries having higher powers and discharge rates at low temperatures. With respect to nickel metal hydride batteries, the barrier to low temperature performance appears to reside primarily in the operating characteristics of the negative hydrogen storage alloy electrode. Consequently, a need exists for improving the performance of hydrogen storage alloys at low temperatures. New concepts in materials design are required to meet this need.
The instant invention includes high porosity hydrogen storage alloys that, when included as the active component of a negative electrode in a nickel metal hydride battery, lead to batteries that provide higher discharge rates and higher powers, especially at low operating temperatures. The instant alloys may also be utilized as thermal hydrogen storage alloys and in fuel cells. The instant invention utilizes the Ovshinsky principles of atomic engineering, chemical modification and total interactive environment to improve the kinetics of hydrogen storage alloys. The improved kinetics achieved with the instant hydrogen storage alloys provide significantly improved low temperature operating characteristics and make high power operation at xe2x88x9230xc2x0 C. practical for the first time.
The instant modified porosity alloys include different forms of base alloys represented by the AB, AB2, AB5 or A2B families of hydrogen storage materials where component A is a transition metal, rare earth element or combination thereof and component B is a transition metal element, Al or combination thereof. Representative examples of component A include La, Ce, Pr, Nd, and combinations thereof including mischmetal. Representative examples of component B include Ni, Co, Mn, Al and combinations thereof. The instant alloys include catalytic metallic particles surrounded by a supporting matrix that has been engineered to improve access of electrochemically and thermally reactive species to catalytic sites, thereby improving kinetics.
In one embodiment, a base alloy is modified with one or more microstructure tuning elements that act to favorably tailor the properties of the supporting matrix to provide a higher concentration of catalytic metallic particles as well as greater accessibility of reactive species to the catalytic metallic particles. The microstructure tuning elements facilitate an accelerated and directed preferential corrosion of the support matrix during activation or operation to provide a more porous and accessible support matrix that also includes locally enriched concentrations of catalytic metallic particles distributed throughout the surface region of the instant hydrogen storage alloys. As the support matrix becomes more porous and less oxidic, the catalytic metallic particles may become at least partially self supporting. The modifications of the support matrix provided by the instant microstructure tuning elements increase the number of catalytic sites and facilitate access of reactants to catalytic sites as well as departure or transport of reaction products from catalytic sites thereby providing faster kinetics for hydriding/dehydriding and charging/discharging processes of thermal and electrochemical hydrogen storage alloys. The instant microstructure tuning elements include Cu, Fe, Al, Zn and Sn.
In another embodiment, the support matrix is made more porous through alloy processing. Control of certain alloy processing parameters (e.g. heat treatment temperature, processing ambient, time of contact with air, electrolyte etc.) increases the porosity of the support matrix. In still another embodiment, inclusion of etching as a step during processing also provides a way to increase the porosity of the support matrix.
In a preferred embodiment, porosity of the support matrix is increased through formation of open channels or voids having a cross sectional dimension of 1-2 nm that extend in three dimensions throughout the surface layer. The channels or voids provide pathways to and from catalytic metallic particles that facilitate access of reactant species to and departure of product species from the catalytic metallic particles. The kinetics of charging/discharging processes and hydriding/dehydriding processes are thereby enhanced.
Electrodes may be formed from the instant high porosity alloys and used as negative electrodes in nickel metal hydride batteries to achieve batteries providing superior power and discharge rates, especially at low temperatures. In one embodiment, a C cell NiMH battery including the instant high surface interface porosity B12 alloy (La10.5Ce4.3Pr0.5Nd1.4Ni64.5Co3.0Mn4.6Al6.0Cu5.4) as the active negative electrode material provides a specific power of about 2000 W/kg at 80% SOC and 35xc2x0 C. In another embodiment, a C cell NiMH battery including the instant high surface interface porosity B12 alloy as the active negative electrode material provides a specific power of 150 W/kg at 50% SOC and xe2x88x9230xc2x0 C. By comparison, a conventional alloy (La10.5Ce4.3Pr0.5Nd1.4Ni60.0Co12.7Mn5.9A4.7) provided a specific power of essentially zero under the same low temperature conditions in the same battery package.